For several decades, electronic devices have been implanted in humans and animals. Such devices are generally used to monitor or regulate the functions of body organs and the like. For example, a pacemaker monitors and regulates heart rate, delivering electrical impulses as required to maintain a satisfactory heart beat. Such implantable devices are powered by batteries which have a finite capacity, and which also present a limitation in terms of available peak power. As a result, there is a significant need to minimize the energy expended during operation of the implantable device, and to reduce the peak power required for special situations such as communications with external devices.
In recent years, implantable electronic device technology has rapidly advanced. Sizes and weights have been dramatically decreasing, while functionality has been increasing. These advances have created a corresponding demand for two-way communication between the implanted electronic device and an external device, generally known as a programmer. For example, in a pacemaker system, the programmer downloads data to the implanted pacemaker, such as operating parameters. Thus, an advantage of modern pacemaker systems is that the physician can re-program various operating parameters, such as rate, pulse level, mode, etc. Likewise, the implantable device up-links data to the programmer for analysis, such as patient data (e.g., average heart rate) or device operational data (e.g., battery voltage). Indeed, the pacemaker is capable of storing significant amounts of data for diagnostic purposes, which data frequently must be transmitted to the programmer for evaluation by the physician. Unfortunately, the increasing need for telemetry has resulted in a corresponding increase in demand on implantable device battery power. The telemetry system uses a significant amount of battery power when operational, which affects lifetime. And potentially more important, the telemetry system may greatly increase the current load on the battery during periods of transmission, to the point where battery peak power availability substantially curtails the possible rate of transfer. Many implantable systems, such as pacing systems, can accumulate and store large amounts of event and operational data, and this creates a need to transfer such data reasonably quickly and efficiently. The more the data transfer rate is limited, the longer the physician is tied up. For these and other reasons, efficient transmitter operation is vitally important to the useful life and communications capability of an implantable device.
Well-known transmitter designs presently in use with implantable devices employ the reflected impedance from a resonating LC circuit to transmit data. In such a design, an external coil is inductively coupled with the coil in the LC circuit within the implanted device. LC circuit transmitters are generally very efficient because of the small amounts of current required to transmit the signal relative to other transmission means, such as RF. However, the bandwidth of such systems has been limited. A basic problem of many such transmitters is that the LC tank resonance frequency and the drive frequency are not inherently matched, and are subject to the accuracy of the components. The coils used are often subject to detuning by metal objects, programmer heads, etc.
In addition to the need for improved efficiency transmitters, there is a need to incorporate improved encoding of a form adaptable to the transmitters, and by which higher data rates can be communicated. One form of data encoding that has come into use is BPSK, biphasic shift keying. By this technique, a portion of the carrier referred to as a "symbol," e.g., a fill cycle of a sine wave, is shifted in phase to one of two states, so as to carry a bit of information. Likewise, for FSK, frequency shift keying, a symbol is shifted in frequency; and for ASK, the amplitude of the symbol is shifted from one level to another. However, present circuits for BPSK, FSK or ASK lack the efficiency desired for implantable-type systems, and are generally lacking in terms of providing high data transfer rates.
In view of the importance of telemetric communications with implanted devices such as pacemakers, a telemetry system that extends the battery life of such devices and/or provides increased bandwidth and greater data transfer rates is very desirable. Such a system would enable longer effective lifetimes for implanted devices, as well as a greater communication capability between the implanted devices and the external programmer. Thus, there is a long-felt need for a more efficient telemetry system for use with implantable devices, and specifically for improved transmitters, data encoding techniques, and decoding techniques.